I am well aware that the facts of organic chemistry seem to demand that the valence electrons be grouped in certain definite equilibrium positions about the periphery of the atom, and that at first sight this demand appears difficult to reconcile with the theory of electronic orbits. But a little reflection shows that there is here no necessary clash. With a suitable orientation of orbits, these localized valencies of chemistry are about as easy to reconcile with an orbit theory as with a fixed electron theory. [Pg 229] Fig. 31—Hypothetical structure of the radium atom [Pg 230] It is only for free atoms that spectroscopic evidence has forced us to build up orbit pictures of the foregoing sort. When atoms unite into molecules, or into solid bodies, these orbits will undoubtedly be very largely readjusted under the mutual influence of the two or more nuclei which are now acting simultaneously upon them. It has been objected, too, that the Bohr theory is not a radiation theory because it gives us no picture of the mechanism of the production of the frequency . This is true, and therein lies its strength, just as the strength of the first and second laws of thermodynamics lies in the fact that they are true irrespective of a mechanism. The Bohr theory is a theory of atomic structure; it is not a theory of radiation, for it merely states what energy relations must exist when radiation, whatever its mechanism, takes place. It is the first attempt to determine in the light of well-established experimental facts what the electrons inside the atom are doing, and as such a first attempt it must be regarded as, thus far, a success, though it has by no means got beyond the hypothetical stage. Its chief difficulty arises from the apparent contradiction involved in a non-radiating electronic orbit, and there appears to be no solution to this difficulty save in the denial of the universal applicability of the classical electromagnetic laws. But why assume the universal applicability of these laws, even in the hearts of atoms, when this is the first opportunity which we have had to test them out in the region of the infinitely small? There is one other very important relation predicted by the Bohr theory and beautifully verified by experiment, but not involving at all its orbital feature. The frequency value of the inmost, or level, [Pg 231] can be exactly determined by measuring the absorption edge so beautifully shown on the De Broglie photographs opposite p. 200. Let us call this frequency . Similarly, to each orbit in the second or quantum state, there corresponds a definite absorption edge . Two of these are shown clearly in Fig. 23. The difference between the absorption frequency and each absorption frequency should obviously, according to Bohr, correspond exactly to the frequency of an emission line in the X-ray spectrum, i.e.,